1. Field of the Invention
This present invention relates to an automatic speed change device for bicycle gearbox, and more particularly to an automatic speed change device, which is employed to control the speed change and the gears shift of the rear wheel upon the treading rate of the user.
2. Description of the Prior Art
With the technology development, human's transportation is with variety and multi-function. Now, most people living in towns or cities employ automobiles or motorcycles as transportation. Bicycle becomes an entertainment or transportation for short distance thereof. Besides, in some smaller towns, most people still prefer employing bicycles as their transportation tools. Originally, a bicycle is driven by a single front gear and a single rear gear. With the development of bicycle, a bicycle in today can be driven with manual speed change mode by adding a rear wheel speed change device. Therefore, most bicycles at present comprise a rear wheel speed change device for the user to manual adjust the gear upon the road condition. Generally, when riding on level ground or the downgrade, the speed change gear set of the rear wheel speed change device has to be shifted to a smaller gear for speeding up. At that time, the treading force of the user is increased, and the treading rate thereof is decreased. Oppositely, when riding on an upgrade, the speed change gear set of the rear wheel speed change device has to be shifted to a greater gear, and thus the rear wheel can be rotated in slower speed but greater power. As so, the treading rate of the user is decreased, and the treading rate can be increased. In other words, the user can pay less effort and get more kinetics by the mentioned design. Therefore, bicycles with the mentioned design are favored by young users as transportation tool when going outside. However, the above-mentioned design is not convenient enough. The rear wheel speed change device is controlled by the allotting device of the speed change gear set, and the allotting device is controlled by the speed change rod disposed at the handle of the bicycle. Hence, if the user is too old or too young to use the speed change function well, the speed change function of the bicycle cannot be applied as its original design.
Hence, it is an important object of developing a structure of automatic speed change device for bicycle gearbox for simplifying the speed change process and making riding more easily for all age.